This invention relates to apparatus for the gravity-cast, bottom-filled, xe2x80x9clost foamxe2x80x9d casting of metal, and more particularly to sprues therefor that reduce porosity and inclusions in the casting.
The so-called xe2x80x9clost foamxe2x80x9d casting process is a well-known technique for producing metal castings wherein a fugitive, pyrolizable, polymeric, foam pattern is covered with a thin, gas-permeable, ceramic coating, and embedded in an unbounded sand mold to form a mold cavity within the sand. Molten metal (e.g., iron or aluminum) is then introduced into the mold cavity to pyrolize the foam pattern, and displace it with molten metal. Gaseous and liquid pyrolysis products escape through the gas-permeable, ceramic coating into the interstices between the unbonded sand particles. The most popular polymeric foam pattern comprises expanded polystyrene foam (EPS) having densities varying from 1.2 to 1.6 pounds per cubic foot. Other pyrolizable, polymeric foams such as polymethylmethacrylate (PMMA), and copolymers are also known. The molten metal may be either gravity cast (i.e., melt is poured from an overhead ladle or furnace) or countergravity cast (melt is forced, e.g., by vacuum or low pressure, upwardly into the mold from an underlying vessel).
In gravity cast lost foam processes, the hydraulic head of the melt is the driving force for filling the mold with melt. Gravity cast lost foam processes are known that (1) top-fill the mold cavity by pouring the melt into a basin overlying the pattern so that the melt enters the mold cavity through one or more gates located above the pattern, or (2) bottom-fill the mold cavity by pouring the melt into the flow channel of an elongated sprue that lies adjacent the pattern and extends from above the mold cavity to the bottom of the mold cavity for filling the mold cavity from the bottom through one or more gates located beneath the pattern. After cooling, the metal left in the sprue and the gate(s) are cut from the casting and recycled. FIGS. 1 through 5 (to be discussed hereinafter) depict various known sprue arrangements for bottom filling lost foam molds. Castings produced by these arrangements suffer from (1) undesirable porosity, (2) folds formed by trapped liquid pyrolysis products (hereafter liquid-induced folds) and/or (3) oxide inclusions in the finished casting resulting from the presence of pyrolizable foam in the sprue""s flow channel. In this regard, when foam in the sprue""s flow channel pyrolizes, gaseous pyrolysis products bubble back up through the melt in the flow channel where they cause considerable turbulence over and above that caused by pouring alone. This extra turbulence causes air, as well as gaseous and liquid pyrolysis products to become entrained in the melt, and carried forward into the mold cavity with the melt, where they cause liquid-induced folds, porosity and oxide inclusions which weaken the casting.
The present invention seeks to reduce the formation of pores, liquid-induced folds and oxide inclusions in bottom-filled, gravity cast, lost foam castings by eliminating pyrolizable foam from the flow channel of the sprue that supplies molten metal to the mold. More specifically, the present invention contemplates apparatus for the bottom-fill, gravity, lost-foam casting of a casting which apparatus comprises: a bed of loose sand forming a mold having a molding cavity therein for shaping molten metal into the casting; a flask containing the bed of sand; a fugitive pattern embedded in the sand and shaping the mold cavity, which pattern has the shape of the casting to be cast and comprises a polymeric foam pyrolizable by the molten metal; a fugitive body attached to the pattern and forming a gating system in the sand for supplying molten metal to the mold cavity, which body has an underside and is comprised of a pyrolizable foam; a downwardly-facing inlet to the gating system for admitting molten metal upwardly into the gating system into contact with the underside of the body; a hollow sprue embedded in the sand for conducting molten metal to the inlet, which sprue is free of pyrolizable foam and made from a material that is not pyrolizable by the molten metal; a mouth at one end of the sprue higher than the pattern for admitting molten metal into the sprue; and an upwardly-facing outlet at the other end of the sprue underlying the gating system and engaging the inlet for directing molten metal from the sprue upwardly into the gating system. Preferably, the sprue is made from a porous, gas-permeable ceramic. Most preferably, the porous ceramic sprue is made from ceramic fibers or particles (e.g., alumina, alumina silicate, silicon carbide, fiberglass, bonded sand, bonded glass spheres, bonded hollow ceramic spheres, and ceramic aggregates).
According to one embodiment, the sprue is L-shaped having a central flow channel through which the melt flows, a vertical leg that receives gravity-poured molten aluminum from an overhead ladle or furnace, and a horizontal leg extending from the vertical leg to beneath the gating system. The mouth that receives the poured melt is atop the vertical leg and the outlet that engages the inlet to the gate is atop the horizontal leg.
Most preferably, the sprue has a J-shaped flow channel having: a first leg that receives molten metal gravity-poured into the sprue and flows it downwardly adjacent the pattern; a second leg, shorter than the first leg, for flowing the molten metal upwardly toward the inlet to the gating system; and a transition section joining the first and second legs for changing the direction of flow of the molten metal between the first leg and the second leg. Preferably, the cross-sectional area of the flow channel transverse the second leg is greater than the cross-sectional area of the transition section between the legs to slow down the rate of advance of the melt front toward the gating system.
The present invention prevents any pyrolysis products from becoming entrained in the melt in the sprue, and insures that any pyrolysis products that are formed are pushed into the gating system and/or molding cavity ahead of the advancing melt front.